1. Field of the Invention
The present invention relates to a bromate ion measuring method and apparatus for measuring the concentration of bromate ions in a test water sample.
2. Description of the Related Art
Bromide ions (Br−) are contained in raw waters, such as river water. When the water is treated by the ozonated process, bromide ions react with ozone to generate bromate ions (BrO3−). It is considered that bromate ions are a carcinogen. Thus, the WHO (World Health Organization) provides that the guideline value of the concentration of bromate ions in drinking water is 10 μg/L. In Japan, a revision was made about a ministerial ordinance concerning water quality standard that was issued on May 30, 2003, and the revised ordinance provides that the standard value of the concentration of bromate ions in drinking water is 10 μg/L.
As a method for measuring the bromate ion concentration in water, known is an ion chromatography with post-column reaction (IC-PC method). The IC-PC method is a method of using an anion exchange column to separate bromate ions in a test water sample, adding sulfuric acid, and a mixed liquid of sodium nitrite and sodium bromide to the eluate of the bromate ions, thereby converting the bromate ions to tribromide ions, and then measuring the ultraviolet absorbance of the tribromide ions to determine the bromate ions quantitatively. In this IC-PC method, two-stage reactions take place, and it is necessary to convert, in the first-stage reaction thereof, bromic acid into tribromide ions with a potassium-bromide/sulfuric-acid solution, and using, in the second-stage reaction, a sodium nitrite solution to ensure the linearity of a calibration curve in a low concentration range thereof. Thus, an operation for measuring the bromate ion concentration according to the IC-PC method is complicated, and it is difficult to apply this method to a process apparatus.
In light of such a background, in recent years, a suggestion has been made about a method of using fluorescence intensity to measure the concentration of bromate ions. In this method, to a test water sample, trifluoperazine (TFP), which is a fluorescent substance that is reactive by the co-existence thereof with bromate ions, and hydrochloric acid are added, the fluorescence intensity is measured at an excitation wavelength of 300 nm and an emission wavelength of 480 nm, and then a calculation is made about the difference in fluorescence intensity between the test water sample and a standard sample containing no bromate ion. From the use of a calibration curve of fluorescence intensity difference and the concentration of bromate ions, and the fluorescence intensity difference of the test water sample calculated above, the bromate ion concentration is measured. According to this method, bromate ions can be easily and rapidly measured with a high precision.
In the meantime, TFP shows a quenching reaction when the excitation wavelength, and the emission wavelength for measurement are 300 nm and 480 nm, respectively. However, when the excitation wavelength and the emission wavelength for measurement are 300 nm and 480 nm, respectively, the optimal concentration of hydrochloric acid that makes it possible to ensure the linearity of the calibration curve is a very high value of 6 N. Therefore, in the conventional method, wherein the hydrochloric acid concentration used for the measurement is high, the apparatus (concerned) is easily corroded, and running costs increase. Furthermore, under this measuring condition, the slope value of the calibration curve is changed by co-existing nitrate ions, so that the bromate ion concentration is not precisely measured in some cases. In light of such a background, it has been expected to provide a technique making it possible to measure the concentration of bromate ions precisely without being affected by coexisting substances while the concentration of hydrochloric acid necessary for the measurement is lowered.